Many piezo-resistive pressure sensors employ a conventional full (4-arm) Wheatstone bridge mechanization, powered by a constant voltage source, to provide a differential voltage output proportional to a pressure sensed. High performance sensors, which may provide accuracy (defined as the sum of the non-compensatable errors) of about 0.012% full scale (FS) or better and long term stability of about 0.02% FS or better over the life of the product, can also include an on-chip full Wheatstone temperature bridge. The temperature output can then be used to compensate and calibrate the pressure output using microprocessor-based electronics, for example. Thus, high performance sensors including both pressure and temperature Wheatstone bridge mechanisms include two separate configurations on the same chip.
Full bridge mechanisms may use ion implanted resistors. For example, the full Wheatstone temperature bridge utilizes a high thermal coefficient of resistance (TCR) light implant element in one set of opposing arms of the Wheatstone bridge and a low TCR heavy implant element in the other set of opposing arms of the Wheatstone bridge that provides an output proportional to temperature.
However, full bridge mechanizations, can be susceptible to non-compensatable errors such as non-ratiometricity errors, power-up drift, thermal hysteresis, and time dependant high temperature induced drift (HTNR). These errors may be related to one or more of the following: a difference in voltage sensitivity of elements in a top of the Wheatstone bridge compared to those in the bottom of the Wheatstone bridge, migration of ionic contaminants in the presence or absence of an electrical field, and the magnitude of the sensor voltage source.
In addition, many sensors also provide on-chip implanted feedback and bias resistors for use with an external operational amplifier. However, this level of complexity can require seven heavy implant elements and up to nine light implant elements for a total of sixteen elements to implement, and a sensor package that requires up to eleven pins with glass-to-metal hermetic seals, all of which add cost to the product.
As a result, existing high performance sensors require complex configurations to enable both pressure and temperature sensing.